Post-register allocation, but before brw_fs_lower_vgrfs_to_fixed_grfs,
we have registers with the VGRF file but they are actually fixed GRFs.
brw_print_instructions_to_file() was seeing VGRFs and trying to access
their size, but using bogus register numbers that could be out-of-bound.
Detect when we're post-RA and avoid doing this.
Reviewed-by: Caio Oliveira <caio.oliveira@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/30530>